General-purpose resins such as polyethylene, polypropylene, polyvinyl chloride and polystyrene produced from petroleum as the starting material have been used in various fields such as daily sundries, household appliances, automobile parts, building materials and food packaging, because they are lightweight and excellent in processability, physical properties and durability. However, the excellent durability of these resin products is problematic in disposal after fulfilling their roles and makes them inferior in degradation in the nature, and thus they can exert an adverse influence on the biological system.
To solve this problem, biodegradable polyester resins such as polylactic acid, lactic acid/other aliphatic hydroxycarboxylic acid copolymers, aliphatic polyesters derived from aliphatic polyvalent alcohols and aliphatic polyvalent carboxylic acids, and copolymers containing these units, have been developed as biodegradable polymers made of thermoplastic resin.
These biodegradable polymers, when placed in soil, seawater or an animal body, initiate degradation in a few weeks by the action of enzymes produced by microorganisms living in the nature and will disappear in about 1 year to several years. Their decomposed products are those nontoxic to humans, such as lactic acid, carbon dioxide, water etc. Among aliphatic polyesters, the polylactic acid-based resin is characterized in that it is produced inexpensively due to production of L-lactic acid in a large amount by fermentation from sugars obtained from potato, sweet potato, etc., its starting material is a natural crop from which the amount of total carbon oxides discharged is very small, and the performance of the resulting polymer is highly rigid and excellent in transparency, and thus the application of the polylactic acid-based resin is expected at present to be promising and is used in flat yarns, nets, horticultural materials, seedling pots etc. in the fields of agriculture and civil engineering, as well as in envelopes with a window, shopping bags, compost bags, writing materials, sundries etc. In the case of polylactic acid, however, its applicable field is limited to rigid moldings because of its brittleness, rigidness, and lack of pliability, and the polylactic acid when formed into a film is problematic due to insufficient flexibility and whitening on bending and is thus not used at present in the field of soft or semi-rigid products. As techniques of application to the field of soft and semi-rigid products, various methods which include adding plasticizers have been proposed. For example, techniques that include adding plasticizers such as tributyl acetyl citrate, diglycerin tetraacetate etc. have been disclosed. Polylactic acid when blended with these plasticizers can attain excellent flexibility upon extrusion molding etc. into a film or sheet; however, there is a problem that the polymer is in a noncrystal state, thus undergoing significant change in flexibility (temperature sensitivity) upon a change in temperature in the vicinity of glass transition point, and is poor in thermal resistance at high temperatures, thus changing physical properties depending on the season and cannot be used under a high-temperature environment. To solve this problem, a method of improving the thermal resistance etc. of polylactic acid, which includes adding a crystal nucleating agent such as talc to crystallize the polylactic acid is proposed (JP-B 3410075).
JP-B 3411168 discloses a method of producing an aliphatic polyester-molded product having both transparence and crystallizability, which includes molding an aliphatic polyester composition containing a transparent nucleating agent such as an aliphatic carboxylic acid amide having a melting point of 40 to 300° C. and then thermally treating it during or after molding.
WO-A 2003/042302 discloses a lactic acid-based polymer composition containing an amide compound having a specific structure, a plasticizer, and a lactic acid-based polymer, as well as a method of producing a molded product thereof.